Idles fast then slowly drops

[sleeptalker]

Hiya,

So I picked up my S3 this morning having had a whole raft of work done on it, including a full 12k service....and unfortunately it's come back to me with an idle speed fault that wasn't there at the start!

In short the idle speed seems to rest at 2k rpm and then over the course of a few seconds slowly drops to 1k rpm. As you can imagine this does smooth out the normally jerky decelleration I used to get (nice!), but also means the car tends to run away a bit in 1st and 2nd gear (not so nice!)

The chap who services my S3 is a really good, knowledgeable mechanic (ex-TVR) and I trust his judgement and he's confirmed that the timing is spot on and that the ECU was put into the correct maintenance mode while they were doing the 12k service.

He also said the hydrocarbon count was quite high from the exhausts and that this may also be a sign of a sensor playing up somewhere in the engine.

I've been reading all the threads about throttle pots but they don't seem to fit what i'm seeing - it's not 'hunting' for revs, it's more just idling quite high and then gradually dropping to what I am used to being normal from 2k -> 1k rpm.

I'm happy to fiddle under the bonnet with a bit of guidance if anyone can suggest what to check?

If I do the obvious things like unplug sensors and clean their contacts etc. do I need to disconnect the battery or be wary of any weird ECU type reset's that would then need to be carried out?

Cheers!

Anders

[Roy C]

Anders,

Check the Air Bypass Valve (AKA Idle Speed Control Valve - acts like a choke). It may just need cleaning inside.

Roy

[sleeptalker]

Will do - looks straight forward enough to get to.

Is the best thing to just spray a load of carb cleaner into it?

[Barkychoc]

It could also be an air leak between the airflow meters and the engine, check the 2 pipes at the front are fitted well and don't have any splits etc.
The other possibility is a knackered brake servo allowing air into the engine. You could try blocking off the hose either by squashing it or pulling it out of the servo and blocking the end. What are the brakes like?

[sleeptalker]

Interesting you say that - I asked they fit a new brake servo for me that I sourced off eBay (looks in good condition mind you and the brakes feel fine since it was fitted.)

I'll check what you say and remove the hose from the servo and pinch it to see if that settles things down.

Thanks!

[Roy C]

sleeptalker said:

Roy C said:

Anders,

Check the Air Bypass Valve (AKA Idle Speed Control Valve - acts like a choke). It may just need cleaning inside.

Roy

Will do - looks straight forward enough to get to.

Is the best thing to just spray a load of carb cleaner into it?

Yep. It's one of those things that never gets cleaned, but at least it will eliminate it from your investigation. You may want/need to change the gasket that joins it to the plenum too.

[Griffinr]

I had two issues on mine that caused the same symptoms. First was a badly fitted vacumm hose to the brake servo and the second was an incorrectly adjusted throttle stop. The throttle butterflies should be closed completely at tickover as all of the air is supplied by the idle air valve.

Good luck with sorting it, it took me ages.

Rob.

[PompeyTim]

Also worth checking the vacuum hose from the fuel pressure regulator to the front of the plenum... don't ask me how I know that this can cause these symptoms!!

[tvrgit]

If the ECU has been reset ( say by the battery being disconnected during the service) then the ECU will have forgotten it's adaptive settings. It maybe needs to relearn. Mine does this every time the battery is disconnected.

Try starting the car, leave it idling till the fan kicks in and goes off again. Switch off and let engine go cold. Next time you use it, it should be fine.

[sleeptalker]

Quick update:

Been tinkering for an hour and tried the following:

Disconnected the battery for an hour then reconnected, started car and the same problem - if you blip the throttle it goes up as expected then falls, then rises again then falls. Idle is high at ~1100rpm

Disconnected the Throttle Pot and started up - no change

Reconnected Throttle Pot

Disconnected ISCV connector while running - no change

Servo vacuum hose seems ok - kinking that while running and blipping throttle, no change.

Any more ideas very welcome!!

Enjoy your weekends.

A

[Gerald-TVR]

Hi Anders

Check that the throttle body butterfies are fully closing. Possibly slacken off the adjustment screw to ensure that. Air bleed into the plenum chamber at tickover should be by the ISCV only.

Norman

[Barkychoc]

If the battery has been off you have to let the engine do a full temperature cycle as per TVRGIT's comments above - it has to learn some settings through this process - we aren't sure exactly what but it does.

[sleeptalker]

Hi

Thanks for your comments and ideas - as i've said many times before this forum and its members is such a wonderful source of information and help!

I've just been out for a couple of hours washing my S3 and fixing a few niggly things and have done as you say and left it idling until the fan cut in then shut it off. I'll let it all cool down then give it a test drive tomorrow and see if it's any better. I also disconnected all plugs I could see and cleaned them up with electrical contact cleaner to be 100% sure there are good connections all round.

Am really happy otherwise, so many bits and pieces have been repaired or replaced in the past week that my S3 is driving and looking the best i've seen it since I bought it last August - S's are such great little cars aren't they?!

Thanks again and i'll let you know how I get on tomorrow!

Enjoy the nice weather!

A

[sleeptalker]

Annoying, but started up the S3 last night and once again it's sustaining revs at 2k then slowly letting them drop.

Any more ideas before I resort to using professional help - both for me and the car ;0) ?

A

[Griffinr]

sleeptalker said:

Annoying, but started up the S3 last night and once again it's sustaining revs at 2k then slowly letting them drop.

A

Have you made sure the throttle butterflies are absolutely shut, it can be quite difficult to acheive as it requires a bit of slack in the throttle cable, slackening off of the roller in the throttle plate and adjustment of the throttle stop. I had several goes at mine before I got it right.

[sleeptalker]

No i've not checked that - i'll read up on it tonight in 'the bible' and if it's not too hard to mess up i'll give it a go!

Will I need any new gaskets or anything like that to do it?

A

[Griffinr]

sleeptalker said:

No i've not checked that - i'll read up on it tonight in 'the bible' and if it's not too hard to mess up i'll give it a go!

Will I need any new gaskets or anything like that to do it?

A

No gaskets required as there is nothing to remove. Check there is a bit of slack in the throttle cable first and that it is returning to the stop properly as it could be as simple as that.
Then if thats not the problem you can back off the throttle stop a bit (it may be covered by a little white cap. You need to slacken the roller that runs in the slot in the throttle plate before you adjust the throttle stop.
I adjusted the stop till it was clear then back up till it just gripped a piece of paper between the stop and the throttle plate.
When you re-tighten the roller make sure it is clear of both sides of the slot, a screw driver with socket attachment works best as it does not apply any side load.

The whole job should only take 20 minutes, is not difficult but does require a bit of care. You require only a screw driver, 8mm open ended spanner and a small socket I can't remember the size of (posibly 10mm).

Rob.

[sleeptalker]

Hi Rob,

Still no joy - I checked the throttle cable has a little slack and it does. I did notice the bent over bit of metal that is the throttle-stop was a little 'too' bent over (over 90 degrees), so I bent it back a little to see if it made any difference. Again no joy. Idle sits at ~ 1100 rpm and when you blip the throttle is goes up as you increase the throttle, then when you release it drops to 'normal' and then revs up from idle for a few seconds, and then drops back to ~1100 rpm.

You should be able to see the throttle assembly in this picture:

http://thumbsnap.com/sc/dfoPWZ4L.jpg

Is there a way of detaching the throttle stop and manually moving the pivot the butterflies sit on, or is it all integral?

If this isn't possible am I right in thinking the next step is to remove the 2 large air hoses and look into the throttle-bodies to check the butterflies?

Anything else I should check first as i'm a bit worried about taking it all apart when the car itself runs perfect otherwise?

Cheers!

A

Edited by sleeptalker on Sunday 20th June 19:45

[Barkychoc]

Does the idle speed hunt up and down or is it just a bit fast and slow to fall?

[sleeptalker]

I wouldn't say it hunts it does this when you blip the throttle: goes up when you depress the throttle, then drops to 'normalish' idle at ~1100rpm then automatically rises to 1500-2000rpm for about 3 seconds, then slowly drops back to ~1100rpm.